In human cardiocytes, the contractile units are linked together by Z bands. Although specific proteins have been localized in Z band regions of some vertebrate muscles, the organization of these proteins into a filament lattice is unknown. Z bands vary in width but the functional significance of this variation remains obscure. Using optical diffraction methods we have been able to analyze Z and bands greater than 90 nm wide in mammalian cardiac and slow skeletal muscle and have identified a common structural unit. Our optical reconstruction studies show that these units are added side-to-side and end-to-end to make a regular arrangement of axial and connecting filaments. However, the entire Z band is rarely a uniform lattice. Optical reconstructions strongly suggest variation in the arrangement of connecting filaments in the Z band at rest length and suggest that the Z band is a dynamic structure. Our purpose in these studies is to further determine the organization of Z band components in both normal and widened Z bands and to relate this organization to proposed functions for Z bands. A systematic approach for Z band analysis using goniometer tilt and stereo techniques for electron microscopy at standard and high voltages, together with optical diffraction and reconstruction methods, will be continued. We will expand our multilevel approach to include studies of Z bands 1) in normal muscle fixed after repeated stimulation and after fatigue; 2) in normal muscle partially extracted and reconstituted; 3) in isolated myofibrils or Z disc sheets and in developing muscle. Additional techniques to be used are computerized image processing, protein isolation and separation and antibody localization. Structural analysis of Z bands in normal and developing muscle should contribute to our understanding of muscle cell function.